It is known to produce polyester polymers through condensation polymerization in the melt phase starting from diacids and/or diesters as one type of monomer and diols as the other type of monomer. In particular, it is known to form polyester polymers based on aromatic dicarboxylic acids such as terephthalic acid (TPA) as the principal diacid and ethylene glycol (EG) as the principal diol with the resulting polymer being called polyester terephthalate (PET) herein. In addition to TPA and EG, PET polymers often contain lesser amounts of other multi-carboxylic acids, e.g. aromatic acids such as isophthalic acid (IPA) and trimellitic acid (TMA), and other multi-alcohols, e.g. 1,4-cyclohexane dimethanol (CHDM) and diethylene glycol (DEG). DEG monomer may be formed in situ from dimerization of EG. Throughout this disclosure and in claims, TPA may be replaced optionally in whole or part by any of its ester derivatives, including but not limited to dimethyl terephthalate (DMT) and bis(2-hydroxyethyl)terephthalate (BHET). Collectively and individually, these various multi-carboxylic acids, multi-esters, and multi-alcohols are termed monomers, as used herein. As used herein, process materials are monomers, catalysts, polymer additives, reaction medium, PET melt product, and various byproducts of the PET formation reactions. Typical byproducts from the formation of PET comprise water, methanol and EG, and various degradation byproducts. Typical degradation byproducts from the formation of PET comprise acetaldehyde, DEG, various dioxanes, and various colored conjugated aromatic molecules. As used herein, reaction medium is a mixture comprising at least a portion of EG mixed with at least a portion of TPA and/or an ester of TPA. Optionally, reaction medium may comprise various catalysts, polymer additives, and byproducts of the PET formation reactions.
The melt-phase synthesis of PET is typically executed at temperatures in excess of 250° C. Such high temperatures are needed both to maintain the reaction medium in a flowing molten state and to promote chemical reaction rates. Generally, the importance of higher temperature increases as the degree of polymerization increases.
It is known that the heat of reaction for formation of the ester bond linkages in PET is only mildly exothermic in comparison to the thermal energy input needed to elevate the monomers to the temperature of the reaction medium and to remove as vapors the byproducts of the condensation reaction, e.g. water, methanol. In a commercial-scale PET melt product synthesis facility, very large amounts of thermal energy are added to process materials, especially to the reaction medium. Very large heat-transfer areas are needed to exchange this thermal energy through conductive, isolating, heat-exchange boundary surfaces, which typically comprise various metals and metal alloys.
The large input of thermal energy during synthesis of PET melt product is typically provided by one or more types of heat-transfer fluid through conductive, isolating, heat-exchange boundary surfaces. The conventional choices for heat-transfer fluids are various relatively high molecular weight organic materials that have relatively low vapor pressures even at temperatures in excess of 300° C. This relatively low vapor pressure, as compared to the vapor pressure of water or of low molecular weight organic materials, ameliorates the mechanical design and cost of the conductive, solid boundary surface needed to isolate the reaction mass from the heat-transfer mass while enabling the transfer of thermal energy. These low-vapor-pressure organic heat-transfer fluids are used in various combinations of liquid and vaporized forms, yielding both sensible heat and latent heat of vaporization to the process materials. Suitable low-vapor-pressure organic heat-transfer fluids are available in the Dowtherm, Therminol, and other commercial product lines; and Dowtherm A and Therminol 66 are preferred embodiments.
Unfortunately, such low-vapor-pressure organic heat-transfer fluids have several drawbacks. The low-vapor-pressure organic heat-transfer fluids have relatively low sensible heat capacity and latent heat of vaporization, especially as compared to water, and thus require relatively large mass flow rates to transfer the required amount of thermal energy into the process materials. Conduit diameters for conveying low-vapor-pressure organic heat-transfer fluids in a large PET melt product synthesis facility may exceed 0.5 m. The low-vapor-pressure organic heat-transfer fluids are flammable and require considerable safety precautions. For example, a “river of fire” can occur when an exchanger tube ruptures inside a fuel-fired furnace where the low-vapor-pressure organic heat-transfer fluid is typically heated. Despite careful selection of the organic molecules and rigorous minimization of dissolved oxygen therein, the low-vapor-pressure organic heat-transfer fluids are challenged on thermal stability in the exchanger tubes of a fuel-fired furnace where they are typically heated. The low-vapor-pressure organic heat-transfer fluids are expensive, with the filling inventory for a world-scale PET melt product synthesis facility costing in excess of one million US dollars. Thus, the combined costs for using low-vapor-pressure organic heat-transfer fluids add considerably to the cost of manufacturing PET melt product: capital cost for fuel-fired furnace; capital cost for large diameter piping, valves, insulation, controls, and pumps; capital cost for fire protection; circulation pump energy consumption; fluid degradation losses; thermal energy losses on large pipe sizes, even with thick insulation; and working capital for fluid inventory.